1. Field of the Invention
The present invention relates to a storage device comprising a probe array and to a method for scanning a storage medium.
2. Description of Background
New storage concepts have been introduced over the past few years. Exploiting the capability of imaging and investigating the structure of materials down to the atomic scale achieved by scanning tunneling microscopy (STM) and atomic force microscopy (AFM), probes having a tip are being introduced for scanning appropriate storage media, where data are written as sequences of bits represented by indentations and non-indentations. According to latest demonstrations, indentations with a diameter of the range of 30-40 nm have been written on appropriate storage media. Hence, these data storage concepts promise ultra-high storage areal densities.
First approaches are disclosed in “High-density data storage using proximal probe techniques” by H. J. Mamin et al., IBM Journal Research Development, Vol. 39, No. 6, November 1995. A single tip of an AFM cantilever is placed in contact with a rotating surface of a polycarbonate storage medium. Indentations or non-indentations written on the storage medium represent bits. Writing on the storage medium is accomplished by heat
With the tip being in contact with the storage medium, the heated tip softens the polymer surface. As a force is applied to bring the tip in contact with the surface, the tip creates a small indentation. A mechanical reading mechanism is adopted. As the tip rides over the surface of the storage medium, a topographic indentation causes a deflection of the cantilever. This deflection is detected using a standard optical sensor.
“Mark” as used herein is understood as physical representation of an information unit. Referring to the storage device according to Mamin et al., marks are indentations and non-indentations for instance. Marks representing data are usually aligned along a track due to the movement of a probe while writing data. Tracking refers to controlling the position of the tip, such that it is always positioned over a track centerline of the track. Mamin et al. fill in servo or tracking marks in the data track at specific locations. These tracking marks are placed among data marks. Data marks and tracking marks are alternating. The tracking marks are used for feedback as to whether the tip is on one side of the track centerline or on the other side and by how much it deviates from the track centerline. The tracking marks cover roughly fifteen percent of the track length.
“High-Density Data Storage Based on the Atomic Force Microscope”, by H. J. Mamin et al., Proceedings of the IEEE, Vol. 87, No. 6, June 1999, discloses another single tip based storage device with a rotating disk as storage medium. A single tip of an AFM cantilever is placed in contact with a rotating surface of a polycarbonate storage medium. Indentations or non-indentations in the storage medium represent bits. Writing is accomplished by heating the tip electrically via two conducting legs, which are connected with the tip. Reading is accomplished with a piezoresistive sensor, sensing the deflection of the cantilever when scanning an indentation. Tracking is accomplished by providing a special cantilever structure: The cantilever is provided with vertical ribs for detecting lateral deflections of the cantilever. These deflections are measured piezoresistively.
“The Millipede—More than one thousand tips for future AFM data storage” by P. Vettiger et al., IBM Journal Research Development, Vol. 44, No. 3, May 2000, shows a data storage device based on a mechanical x-/y-scanning of a storage medium with an array of probes each having a tip. The probes scan associated fields of the storage medium in parallel, so high data rates can be achieved. The storage medium comprises a thin polymethylmethacrylate (PMMA) layer. The probes scan the polymer layer in a contact mode. The contact mode is achieved by applying small forces to the probes so that the tips of the probes can touch the surface of the storage medium. Spring cantilevers carry the sharp tips on their end section. Indentations or non-indentations in the polymer layer represent bits. The cantilevers respond to these topographic changes in the surface.
Indentations are written on the polymer surface by thermomechanical recording, where the local probe is heated with a current or voltage pulse during the contact mode, so that the polymer layer is softened locally where the tip touches the polymer layer. The result is a small indentation in the layer, the indentation having nanoscale diameter. Reading is also accomplished by a thermomechanical concept. The heater cantilever originally used only for writing is given an additional function of a thermal reading sensor by using its temperature dependent resistance. For reading purposes, the resistor is operated at a temperature that is not high enough to soften the polymer as is necessary for writing. The thermal sensing is based on the fact that the thermal conductance between the probe and the storage substrate changes when the probe is moving into an indentation, as the heat transport will be more efficient. Consequently the heater's temperature and hence its resistance will decrease. Thus, changes of the continuously heated resistor are monitored while the cantilever is scanned over a corresponding data field.
Applicant's U.S. Pat. No. 5,835,477 discloses a storage device according to Vettiger et al. with a recommendation for rewriting such a storage device. The storage device comprises a circuit for distinguishing between information which is to be erased from a first section of the storage medium and information which is not to be erased in this section. The information not to be erased is copied into another section of the storage device. Afterwards, the first section can be erased. U.S. Pat. No. 5,835,477 further suggests special guides for tracking purposes, arranged at the edges of the storage medium and interacting with mechanical guiding means of the local probe array. These mechanical means might be replaced by optical means.
With marks so densely packed, accurate scanning becomes a critical issue. Accordingly, it would be desirable having a storage device and a method for scanning a storage medium available with high accuracy in scanning and a low consumption of storage area (i.e., a high storage density) for achieving highly accurate scanning.